The present invention relates to a method and an apparatus for ejecting droplets of ink in an ink jet printer. More particularly, it relates to a method and an apparatus for ejecting droplets of ink in which the droplets are ejected by vibrating the ink.
A drop-on-demand type ink jet printer ejects droplets of ink from an ink reservoir to form dots on a printing medium corresponding to the image to be printed. A method of ejecting droplets of ink by using such a drop-on-demand type ink jet printer is disclosed in U.S. Pat. No. 2,512,743. In this known method, a plurality of droplets are sprayed so as to form one dot. Therefore, it is difficult to form a fine dot. Also, the manner of the ejection of the droplets does not constantly correspond to the frequency of the acoustic wave for generating pressure for ejecting the droplets.
An ink jet printer which forms each dot with one droplet is disclosed in U.S. Pat. No. 3,946,398. In this printer, pressure for ejecting a droplet of ink is constantly applied to the ink until a droplet is ejected from the nozzle of the printer. Therefore, the interval between ejections is long and it is difficult to achieve high speed printing.
Another ink jet printer which forms each dot with one droplet of ink is disclosed in U.S. Pat. No. 3,683,212. The attenuation time for the vibration of the ink at the nozzle of this printer is long. Therefore, the interval between ejections is long and it is difficult to achieve high speed printing.
A device for minimizing the attenuation time for vibration of the ink is disclosed in Japanese Patent Publication No. 54-32572. However, the construction of this device is complicated and it is difficult to apply this device to a multi-nozzle structure.